A map of molecular distances between mutations of bacteriophage T4D.

map of physical distances along the complete T4 DNA molecule has been A reported by MOSIG (1 968). This map was obtained using T4 phage particles, which contain considerably less DNA than a complete genome (petit particles). Each of these particles contains a continuous and probably random permutation of the T4 DNA molecule. MOSIG estimated the ratio of petit particles which carried a particular am+ allele and a reference r l . rZZ, or rZZZ marker, to those which carried only the am+ allele. Knowing the length of the DNA contained by the petit particles, the ratio of am+ to a n i f r permits calculation of the distance of the am+ allele to the r mutation. From different crosses, the distances of different am+ alleles to the r reference markers were calculated. Although this method gave consistent results, it has some disadvantages: As the results of different crosses are being compared, the order and distance apart of closely linked markers is difficult to determine; it cannot be used to measure distances within a gene as the petit particle has to carry the whole am+ gene in order to complement the a m helper phage; the petit genome must have a uniform known length or inconsistencies result (MOSIG 1966, 1968). In this paper, a method will be described in which petit particles were used but all the amber mutations to be mapped were carried by one helper phage. The genotypes of individual petit particles were determined by locating each end of their genomes between two amber markers of the helper phage. A map was constructed by assuming that the number of ends between any two markers is proportional to the physical distance separating them. The principal advantage of this method is that all the measurements come from one cross in which the only gene of the petit particle needed to function is rZZB. It can therefore be used to measure distances within a gene and gives an unambiguous order of markers even when two are located in the same gene. Furthermore the method is independent of the length of the DNA in the petit particles, which need not be uniform.